Incomplete ABAP types are types that do not specify all the attributes of a data type, such as the length, the number of decimal places, or the value range. Incomplete types can only be used for the typing of field symbols and formal parameters, not for the definition of data objects or constants. Incomplete types can be either predefined or user-defined1.
The following are incomplete ABAP types:
* C. C is a type for character strings with a generic length. The length of the character string has to be specified when a data object or a constant is defined with this type. For example, DATA text TYPE c LENGTH 10 defines a data object named text with a type c and a length of 10 characters2.
* D. P is a type for packed numbers with a generic length and a generic number of decimal places. The length and the number of decimal places of the packed number have to be specified when a data object or a constant is defined with this type. For example, DATA amount TYPE p LENGTH 8 DECIMALS 2 defines a data object named amount with a type p, a length of 8 bytes, and 2 decimal places3.
The following are not incomplete ABAP types, because they specify all the attributes of a data type:
* A. String is a type for variable-length character strings. The length of the character string is determined at runtime and can vary from 0 to 2,147,483,647 characters. The length does not have to be specified when a data object or a constant is defined with this type. For example, DATA text TYPE string defines a data object named text with a type string and a variable length4.
* B. T is a type for time values in the format HHMMSS. The length of the time value is fixed at 6 characters and does not have to be specified when a data object or a constant is defined with this type.
* For example, DATA time TYPE t defines a data object named time with a type t and a length of 6 characters.
References: 1: Generic ABAP Types - ABAP Keyword Documentation 2: C - ABAP Keyword Documentation 3: P - ABAP Keyword Documentation 4: String - ABAP Keyword Documentation : T - ABAP Keyword Documentation

On-stack developer extensibility is a type of extensibility that allows you to create development projects directly on the SAP S/4HANA Cloud technology stack. It gives you the opportunity to develop cloud-ready and upgrade-stable custom ABAP applications and services inside the SAP S/4HANA Cloud, public edition system. You can use the ABAP Development Tools in Eclipse to create and deploy your on-stack extensions.
On-stack developer extensibility is suitable for the following kinds of applications:
* Applications that provide APIs for side by side SAP BTP apps. On-stack developer extensibility allows you to create OData services or RESTful APIs based on CDS view entities or projection views. These services or APIs can expose SAP S/4HANA data and logic to other applications that run on the SAP Business Technology Platform (SAP BTP) or other platforms. This way, you can create a loosely coupled integration between your SAP S/4HANA system and your side by side SAP BTP apps.
* Applications that access SAP S/4HANA data using complex SQL. On-stack developer extensibility allows you to use ABAP SQL to access SAP S/4HANA data using complex queries, such as joins, aggregations, filters, parameters, and code pushdown techniques. You can also use ABAP SQL to perform data manipulation operations, such as insert, update, delete, and upsert. This way, you can create applications that require advanced data processing and analysis on SAP S/4HANA data.
The other kinds of applications are not suitable for on-stack developer extensibility, as they have different requirements and challenges. These kinds of applications are:
* Applications that integrate data from several different systems. On-stack developer extensibility is not meant for creating applications that integrate data from multiple sources, such as other SAP systems, third-party systems, or cloud services. This is because on-stack developer extensibility does not support remote access or data replication, and it may cause performance or security issues. For this kind of
* applications, you should use side by side extensibility, which allows you to create applications that run on the SAP BTP and communicate with the SAP S/4HANA system via public APIs or events.
* Applications that run separate from SAP S/4HANA. On-stack developer extensibility is not meant for creating applications that run independently from the SAP S/4HANA system, such as standalone apps, microservices, or web apps. This is because on-stack developer extensibility requires a tight coupling with the SAP S/4HANA system, and it may limit the scalability, flexibility, and portability of the applications. For this kind of applications, you should use side by side extensibility, which allows you to create applications that run on the SAP BTP and leverage the cloud-native features and services of the platform.
References: Developer Extensibility in SAP S/4HANA Cloud ABAP Environment, SAP S/4HANA Extensibility - Simplified Guide for Beginners

Based on the following code, the variable type of connection_full is a structure. A structure is a complex data type that consists of a group of related data objects, called components, that have their own data types and names. A structure can be defined using the TYPES statement or based on an existing structure type, such as a CDS view entity or a CDS DDIC-based view. In this case, the variable connection_full is declared using the TYPE addition, which means that it has the same structure type as the CDS view entity /DMO/I_Connection.
The CDS view entity /DMO/I_Connection is a data model view that defines a data model based on the database table /DMO/Connection. The CDS view entity /DMO/I_Connection has the following components:
carrid, connid, airpfrom, airpto, distance, and fltime. Therefore, the variable connection_full has the same components as the CDS view entity /DMO/I_Connection, and each component has the same data type and length as the corresponding field in the database table /DMO/Connection.
References: CDS Data Model Views - ABAP Keyword Documentation, DATA - ABAP Keyword Documentation, Structure Types - ABAP Keyword Documentation

Core Data Services (CDS) are preferable to the classical approach to data modeling for several reasons, but two of them are:
* They implement code pushdown. Code pushdown is the principle of moving data-intensive logic from the application server to the database server, where the data resides. This reduces the data transfer between the application server and the database server, which improves the performance and scalability of the application. CDS enable code pushdown by allowing the definition of semantic data models and business logic in the database layer, using SQL and SQL-based expressions1.
* They transfer computational results to the application server. CDS allow the application server to access the data and the logic defined in the database layer by using Open SQL statements. Open SQL is a standardized and simplified subset of SQL that can be used across different database platforms. Open SQL statements are translated into native SQL statements by the ABAP runtime environment and executed on the database server. The results of the computation are then transferred to the application server, where they can be further processed or displayed2.
References: 1: ABAP - Core Data Services (ABAP CDS) - ABAP Keyword Documentation 2: Open SQL - ABAP Keyword Documentation